The trouble is that most people who say this end up doing 6 things that take 200 ms each (with the keyboard) instead of 1 thing that takes 1 second (with the mouse), and feel like they're zooming along in hyperspace. You feel like a leet hacker, but are you really getting anything done more efficiently?
Maybe not, but is that 200ms improvement when using the mouse really worth it if i get all warm and fuzzy-feeling by being able to type something? :p Intangible benefits, yo.
More seriously though, i also feel (personally -- maybe i'm an outlier) like switching from keyboard to mouse and back is a bit of a performance hit, so i tend to concentrate my workflow around the keyboard as much as possible. And yeah, maybe (99% certainly!) i'm not the leetest or fastest h4x0r out there, but at least i'm enjoying myself :).
(i would be curious about an actual test case though -- maybe someone could design a few common tasks to complete using one's favourite HID and compare!)
I don't do this because i'm a security expert (i'm not), i just love speed (so AJAX and fancypants websites are out for me, i'll use the terminal or Emacs where possible) and the keyboard.
EDIT: Oh yeah, and i'd never trust my web browser with my PGP key...